Such a method of rerouting includes a plurality of switching nodes with respective addresses and interconnected by respective links used for setting up routes, and said method including the steps of:
detecting said failure of said route on one of said links between first and second said switching nodes in at least said first switching node; PA1 transmitting from said first switching node and on each but said one of said links connected to it, a request message having first and second address fields containing addresses of said first and second switching nodes, respectively; PA1 in each of said switching nodes having received said request message on a link, retransmitting said request message on each link connected to it but said link on which said request message was received, until an alternative route for said data stream is found between two end switching nodes of said alternative route; PA1 allocating said data stream to said alternative route and transmitting it thereon. PA1 broadcasting trigger messages in a controller node; PA1 in each of said switching nodes having received a said trigger message on a receipt link, transmitting to said controller node an information message containing the amounts of bandwidth available on the links connected to it, and rebroadcasting said trigger message on each link connected to it but said link on which said trigger message was received; PA1 in said controller node, calculating from said received amounts of bandwidth an alternative route for said data stream and thereupon allocating said route to said data stream; PA1 transmitting said data stream on said alternative route. PA1 in a said switching node having detected said failure transmitting control messages having a control field for containing the address of said controller node and which is initialized with the address of said detecting switching node; PA1 in each switching node receiving a said control message, transmitting an adapted said control message whose control field is adapted according to a predetermined rule.
Such a method is already known in the art, e.g. from the article `A fast distributed network restoration algorithm` by C. E. Chow e.a., published in the Proceedings of the 12th International Phoenix Conference on Computers and Communications, March 23-26, 1993, Tempe, and more specifically from section III.B thereof. The method described therein applies to synchronous networks and provides one or more alternative routes for data streams previously routed via a link on which afterwards a failure occurred. The two switching nodes at the ends of such a failed link both detect this failure, thus each operating as the above first switching node, and are then arbitrarily designated as so-called Black- and Gray-origin nodes, respectively. These two switching nodes thereupon start transmitting so-called Black and Gray request messages, respectively, on all their outgoing links which have spare channels, each such request message including information as to the identity and the number of data streams for which alternative routes have to be provided. Further switching nodes, referred to in the article as intermediate nodes, receiving such a Black or Gray request message, become designated as Black or Gray nodes, respectively, and retransmit the received request messages, so that eventually Black request messages are received in Gray nodes and Gray request messages are received in Black nodes. Appropriate cross-connections are then made whereby an alternative route for at least part of the data streams to be rerouted is found, and the respective request messages are further transmitted along the respective paths so that Gray request messages finally arrive in the Black-origin node and Black request messages in the Gray-origin node. The Black-origin node then transmits an acknowledgement message to the Gray-origin node, whereas the Gray-origin node starts listening for this acknowledgement message. The Gray-origin node then chooses which of the data streams to reroute via the found alternative route and, upon receipt of the acknowledgement message, it sends a confirm message to the Black-origin node containing the identity of the latter data streams, together with an identity of the alternative route. Upon receipt of this message the Black-origin node makes the appropriate connections and the data stream is transmitted over the alternative route.
The above method is thus based on the insight that a fast recovery or restoration may be obtained by letting request messages originate in the two switching nodes on both sides of the failed link. However, the method does not apply to failures of switching nodes, nor does it apply when one of the two switching nodes on both sides of the failed link does not detect this failure.